1. Field of the Invention
The present invention relates to a dual mode band-pass filter for use in, for example, communications equipment operating in the range of the microwave band to the milliwave band.
2. Description of the Related Art
Conventionally, different types of dual mode band-pass filters have been proposed as a band-pass filter for use in a high frequency band as described in xe2x80x9cMINIATURE DUAL MODE MICROSTRIP FILTERSxe2x80x9d, J. A. Curtis and S. J. Fiedziuszko, 1991 IEE MTT-S Digest.
FIGS. 48 and 49 are schematic plan views illustrating conventional dual mode band-pass filters, respectively.
In a band-pass filter 200 shown in FIG. 48, a circular conductive film 201 is provided on a dielectric substrate (not shown). Input-output coupling circuits 202 and 203 are coupled to the conductive film 201 so as to define an angle of 90xc2x0 relative to each other. A tip-open stub 204 is disposed on the conductive film 201 at a position that defines a central angle of 45xc2x0 relative to the input-output coupling circuit 203. Thereby, two resonance modes with different resonance frequencies are coupled together. Thus, the band-pass filter 200 is configured so as to operate as a dual mode band-pass filter.
In a dual mode band-pass filter 210 shown in FIG. 49, a substantially square conductive film 211 is disposed on a dielectric substrate. Input-output coupling circuits 212 and 213 are connected to the conductive film 211 so as to define an angle of 90xc2x0 relative to each other. The corner of the conductive film 211 in the position thereof defining an angle of 135xc2x0 with respect to the input-output coupling circuit 213 is cut away. The resonance frequencies in two resonance modes are made different from each other by the cut portion 211a, so that the two resonance modes having different resonance frequencies are coupled to each other. Thus, the band-pass filter 210 can be operated as a dual mode band-pass filter.
On the other hand, there has been proposed a dual mode band-pass filter that contains a ring-shape conductive film instead of the circular conductive film (Japanese Unexamined Patent Application Publication No. 9-139612 and No. 9-162610). In particular, a dual mode filter is disclosed, in which a ring-shaped ring transmission line is used, input-output coupling circuits are arranged so as to define a central angle of about 90xc2x0 similarly to the dual mode band-pass filter 200 shown in FIG. 48, and a tip-open stub is provided in a portion of the ring transmission line.
Moreover, Japanese Unexamined Patent Application Publication No. 6-112701 discloses a dual mode band-pass filter that uses a ring transmission line similar to the above-mentioned transmission line. As shown in FIG. 50, in the dual mode filter 221, a ring resonator includes a ring conductive film 222 disposed on a dielectric substrate. In this case, four terminals 223 to 226 are disposed on the ring conductive film 222 so as to define an angle of 90xc2x0 relative to each other with respect to the center of the ring conductive film 222. Two of the four terminals arranged at the positions defining an angle of 90xc2x0 relative to each other with respect to the center of the ring conductive film are connected to input-output coupling circuits 227 and 228, respectively. The remaining two terminals 225 and 226 are connected to each other via a feedback circuit 230.
Moreover, it is described that in the ring resonator including one strip line and having the above-described configuration, perpendicular resonance modes, which are not coupled to each other, are generated, and the coupling degree is controlled by the above-mentioned feedback circuit 230.
In the conventional dual mode band-pass filters shown in FIGS. 48 and 49, a two step band-pass filter can be formed by forming one conductive film pattern. Accordingly, the band-pass filter can be miniaturized.
However, the dual mode band-pass filters each have the configuration in which the input-output coupling circuits, which are separated from each other by a particular angle, are coupled to each other in the circular or square conductive film pattern. Therefore, the dual mode band-pass filters have the disadvantage that the coupling degree cannot be increased, and a wide pass band cannot be achieved.
In the band-pass filter shown in FIG. 48, the conductive film 201 is restricted to a circular shape. In the band-pass filter shown in FIG. 49, the conductive film 211 is also limited to a substantially square shape. Thus, there is the problem that the design flexibility is low.
Dual mode band-pass filters 221 using such a ring resonator as described in Japanese Unexamined Patent Application Publication Nos. 9-139612 and 9-162610 have the problem that it is difficult to improve the coupling degree, and the shape and size of the ring resonator are restricted.
On the other hand, in the dual mode band-pass filter 221 described in Japanese Unexamined Patent Application Publication No. 6-112701, the coupling degree is controlled, and the band-width can be widened by use of the feedback circuit 230. However, in the conventional dual mode filter, the feedback circuit 230 is required. Thus, the circuit configuration becomes complicated. Furthermore, the shape and size of the ring resonator are limited to a ring-shape, so that the design flexibility is very low.
In order to overcome the problems described above, preferred embodiments of the present invention provide a dual mode band-pass filter that is miniaturized and has a greatly improved coupling degree that is easily adjusted and a very wide pass band, while also having very high design flexibility.
According to a preferred embodiment of the present invention, a dual mode band-pass filter includes a dielectric substrate having first and second main surfaces, a metallic film having an opening for coupling two resonance modes and disposed in the first main surface of the dielectric substrate or inside of the dielectric substrate, at least one ground electrode disposed on the second main surface of the dielectric substrate or inside of the dielectric substrate, so as to be opposed to the metallic film through a dielectric layer, and a pair of input-output coupling circuits connected to different portions of the metallic film. With the above-described unique configuration, one of the two resonance modes, that is, one propagated substantially parallel to an imaginary straight line passing through the connection points at which the pair of the input-output coupling circuits are connected to the metallic film, and the other propagated substantially perpendicularly to the imaginary line, is affected by the opening so that the resonance frequency is varied. In other words, the opening is arranged to exert an influence over the resonance current of one of the resonance modes whereby the one resonance mode can be coupled to the other resonance mode. Thus, the opening causes the two resonance modes to be coupled to each other, and as a result, the filter can be operated as a dual mode band-pass filter.
Preferably, the opening has a shape containing a longitudinal dimension and a width dimension.
Also preferably, the plan shape of the opening is a rectangle, an ellipse, or a configuration including a rectangle or ellipse having a bent portion thereof elongating in a direction intersecting the longitudinal dimension thereof.
It is also preferred if the plan shape of the opening is a rectangle, a rhombus, a regular polygon, a circle, or an ellipse.
In addition, a plurality of openings may be formed.
Preferably, the metallic film is disposed on the first main surface of the dielectric substrate, and the ground electrode is disposed on the second main surface of the dielectric substrate.
Also preferably, the metallic film is disposed at a vertical level inside of the dielectric substrate, and the ground electrodes are disposed on the first and second main surfaces of the dielectric substrate, whereby the band-pass filter has a tri-plate structure.
For the purpose of illustrating the present invention, there is shown in the drawings several forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments below with reference to the attached drawings.